Electrical loads must be protected.
Different fuses are used for this purpose depending on the type of supply network and the type of load.
Particularly in direct-current networks, disconnection poses a substantial problem because, unlike in alternating-current networks, no periodic zero crossings are present, so possible switching arcs are not quenched automatically.
In the past, many types of fuses have been designed with elaborate methods for suppressing electric arcs.
Existing fuses are designed to switch in the event of an overcurrent.
However, there is increasing demand for fuse elements that can also be reliably tripped in the event of a moderate current as well.
Existing fuses switch off reliably only in the presence of greatly elevated currents. This is due to the manner in which they are tripped. Specifically, if the protection level is set too low in existing fuses, the fuse is tripped even in the event of momentary overcurrent, such as when a capacitative load is charged or at engine startup, for example. For this reason, existing fuses tend to be generously (over-)dimensioned with respect to overcurrent.
On the other hand, more and more applications are arising in which a continuous slight overload is present which, while hazardous, is not identified as overcurrent.
In networks with limited short circuits, such as PV (photovoltaic) systems, for example, in which the operating current is only about 10% below the short-circuit current, the currents are so small in the event of a short circuit that normal fuses are not tripped.
In PV and wind turbine generators, there is the added difficulty that the currents in partial-load operation (e.g., partly cloudy, moderate wind) lie so far below the maximum current of the system that a short-circuit current that occurs then lies in the range and below the rated current value of the corresponding fuse.